linux-kernel - Re: [PATCH v7 00/30] NT synchronization primitive driver

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Message-ID: <2025010738-amicably-art-f746@gregkh>
Date: Tue, 7 Jan 2025 18:06:03 +0100
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: Elizabeth Figura <zfigura@...eweavers.com>
Cc: Arnd Bergmann <arnd@...db.de>, Jonathan Corbet <corbet@....net>,
	Shuah Khan <shuah@...nel.org>, linux-kernel@...r.kernel.org,
	linux-api@...r.kernel.org, wine-devel@...ehq.org,
	André Almeida <andrealmeid@...lia.com>,
	Wolfram Sang <wsa@...nel.org>,
	Arkadiusz Hiler <ahiler@...eweavers.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Andy Lutomirski <luto@...nel.org>, linux-doc@...r.kernel.org,
	linux-kselftest@...r.kernel.org,
	Randy Dunlap <rdunlap@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
	Waiman Long <longman@...hat.com>, Boqun Feng <boqun.feng@...il.com>
Subject: Re: [PATCH v7 00/30] NT synchronization primitive driver

On Fri, Dec 13, 2024 at 01:34:41PM -0600, Elizabeth Figura wrote:
> This patch series implements a new char misc driver, /dev/ntsync, which is used
> to implement Windows NT synchronization primitives.
> 
> NT synchronization primitives are unique in that the wait functions both are
> vectored, operate on multiple types of object with different behaviour (mutex,
> semaphore, event), and affect the state of the objects they wait on. This model
> is not compatible with existing kernel synchronization objects or interfaces,
> and therefore the ntsync driver implements its own wait queues and locking.
> 
> This patch series is rebased against the "char-misc-next" branch of
> gregkh/char-misc.git.
> 
> == Background ==
> 
> The Wine project emulates the Windows API in user space. One particular part of
> that API, namely the NT synchronization primitives, have historically been
> implemented via RPC to a dedicated "kernel" process. However, more recent
> applications use these APIs more strenuously, and the overhead of RPC has become
> a bottleneck.
> 
> The NT synchronization APIs are too complex to implement on top of existing
> primitives without sacrificing correctness. Certain operations, such as
> NtPulseEvent() or the "wait-for-all" mode of NtWaitForMultipleObjects(), require
> direct control over the underlying wait queue, and implementing a wait queue
> sufficiently robust for Wine in user space is not possible. This proposed
> driver, therefore, implements the problematic interfaces directly in the Linux
> kernel.
> 
> This driver was presented at Linux Plumbers Conference 2023. For those further
> interested in the history of synchronization in Wine and past attempts to solve
> this problem in user space, a recording of the presentation can be viewed here:
> 
>     https://www.youtube.com/watch?v=NjU4nyWyhU8
> 
> 
> == Performance ==
> 
> The performance measurements described below are copied from earlier versions of
> the patch set. While some of the code has changed, I do not currently anticipate
> that it has changed drastically enough to affect those measurements.
> 
> The gain in performance varies wildly depending on the application in question
> and the user's hardware. For some games NT synchronization is not a bottleneck
> and no change can be observed, but for others frame rate improvements of 50 to
> 150 percent are not atypical. The following table lists frame rate measurements
> from a variety of games on a variety of hardware, taken by users Dmitry
> Skvortsov, FuzzyQuils, OnMars, and myself:
> 
> Game                            Upstream        ntsync          improvement
> ===========================================================================
> Anger Foot                       69              99              43%
> Call of Juarez                   99.8           224.1           125%
> Dirt 3                          110.6           860.7           678%
> Forza Horizon 5                 108             160              48%
> Lara Croft: Temple of Osiris    141             326             131%
> Metro 2033                      164.4           199.2            21%
> Resident Evil 2                  26              77             196%
> The Crew                         26              51              96%
> Tiny Tina's Wonderlands         130             360             177%
> Total War Saga: Troy            109             146              34%
> ===========================================================================
> 
> 
> == Patches ==
> 
> The intended semantics of the patches are broadly intended to match those of the
> corresponding Windows functions. For those not already familiar with the Windows
> functions (or their undocumented behaviour), patch 27/28 provides a detailed
> specification, and individual patches also include a brief description of the
> API they are implementing.
> 
> The patches making use of this driver in Wine can be retrieved or browsed here:
> 
>     https://repo.or.cz/wine/zf.git/shortlog/refs/heads/ntsync7
> 

Given a lack of complaints, I've now applied this to my testing tree.
Thanks for sticking with it!

greg k-h